Stands for "Red Green Blue Alpha."
RGBA is a color model based on the RGB model. It mixes red, green, and blue light to create any color in the spectrum, then adds a fourth value, alpha, for transparency. It is a standard color model often used for image editing and web graphics that require gradual levels of transparency.
The RGBA color model, like RGB, is additive. When the three color channels are at their maximum values, they produce white light; at their minimum values, they create black. The alpha channel adds a separate value for transparency, ranging from 0% (completely transparent) to 100% (completely opaque). A transparency value in between results in a pixel that lets some background color show through. For example, an image with a drop shadow uses a gradient of increasingly-transparent pixels to create a shadow that darkens the background behind it.
The most common color depth for RGBA images is 32 bits per pixel: 8 bits each for red, green, blue, and alpha. Each 8-bit color channel can contain a value between 0 and 255; this produces an image with the same number of colors as a 24-bit True Color RGB image (256 x 256 x 256 = 16,777,216 possible colors) with an additional 256 levels of transparency possible for each pixel. However, while image editors display each RGB value as a range between 0 and 255, transparency values are instead shown as a range between 0 and 100%.
CSS uses RGBA values as one possible method of setting the color of text and graphics. Red, green, and blue values are input as numbers between 0 and 255, and alpha values as a number between 0.0 and 1.0. For example, the following value produces fully opaque blue text:
rgba(0, 0, 255, 1.0)
This next example instead produces partially transparent blue text:
rgba(0, 0, 255, 0.5)
NOTE: The .PNG and .TIFF file formats support 32-bit RGBA images, as do professional image editor formats like .PSD. While the .GIF file format supports transparency, it does not include an alpha channel but instead defines a single color in its lookup table as transparent.